Laser system with external optical feedback and use of such system in the graphical industry

ABSTRACT

A diode laser system providing a high-power laser beam with good spatial coherence, which can be focused to a small spot size over long distances and which has a good pointing stability. The laser system comprises a laser diode ( 301 ) adapted to emit a first light beam, where the first light beam comprises a plurality of spatial modes, selection means ( 304 ) adapted to select a predetermined part of the first light beam, and a reflector ( 306 ), where the laser diode and the reflector define a cavity and where the reflector is adapted to reflect the selected part of the first light beam back into the laser diode as a second light beam. The laser system is characterised in that the selection means is adapted to select a part of the first light beam corresponding to a spatial mode with a higher mode number than a spatial mode with maximum gain.

This invention relates to an improvement of the spatial coherenceproperties of laser diode systems.

For many applications it is desirable to use lasers with high outputpower and good spatial coherence properties.

Laser diodes are well known as reasonably priced, small and robustsources of laser beams. Conventional laser diodes with small outputpower and good coherence properties have been available, and they areused in many applications such as CD players, bar-code readers etc.

More recently laser diodes with several Watts of output power havebecome available. These high-power laser diodes are potentiallyapplicable in other areas such as the exposure of print plates in thegraphical industry. However, these laser diodes have a largelight-emitting area of typically 1–2 μm×100–500 μm and, consequently,they have poor spatial coherence properties in the lateral direction ofthe light-emitting aperture, the so-called low-coherency axis. Due tothis disadvantage, the resulting light beam cannot be focused to a smallspot size over long distances. A measure of quality used in connectionwith light sources is the so-called M² value. The M² value is related tothe ability of a light source to be focused.

The international patent application WO98/15994 discloses an externalcavity micro laser system comprising a multimode microlaser and anexternal cavity including the micro laser, a return section and anoutput section. The output section comprises a spatial filter forselecting a portion of the transverse distribution of the laser beam andan output coupler which includes feedback means for causing a fractionof the energy of the selected portion to be fed back into the laser foramplification in the return section of the cavity.

However the above prior art system involves the disadvantage that itdoes not emit sufficient power with a sufficiently low value of M²,preferably smaller than 2, to make it applicable in areas that require ahigh-power beam which can be focused well even over long distances. Anexample for such an application is the exposure of material, for exampleprint plates, in internal drum image setting machines, external drumimage setting machines, flatbed image setting machines or the like.

These applications require high-power laser beams, which can be focusedto a small spot over long distances, which have a good pointingstability, and whose wavelength match the sensitivity of the material tobe exposed.

A simple replacement of the laser diode with one that has a higheroutput power, for example 4W, would not solve the disadvantage of theprior art systems, because high-power laser diodes have a broaderemission angle and a considerably broader frequency spectrum, and thuspoor temporal coherence properties, and a high M² value of for examplebetween 30–500. Furthermore, they have a poor pointing stability.

It is a further disadvantage of the above prior art system that it isdifficult to align.

From WO 9856087 it is known to replace a conventional reflector, such asa mirror or grating, by an adaptive light feedback, such as aphase-conjugate mirror, which may adapt to counteract various types ofchanges imposed on the system and thus may stabilize the system.

However, it is a disadvantage of the above prior art system thatadaptive light feedback systems are expensive.

Therefore, it is an object of the present invention to provide alow-cost laser system for emitting high-power light that can be focusedto a small spot size over long distances.

It is another object of the present invention to provide a laser systemwith good spatial coherence.

It is yet a further object of the present invention to provide a lasersystem for emitting a laser beam with a small value of M².

It is another object of the present invention to provide a laser systemthat is easy to align and a method of aligning.

It is a further object of the present invention to provide a lasersystem with a high pointing stability of the emitted beam, and a methodof adjusting.

It is a further object of the present invention to provide a lasersystem with a good long-term power stability of the emitted beam, and amethod of adjusting.

The spatial characteristics of the light emitted from broad area lasers,laser diode arrays or the like are known to comprise a superposition ofa plurality of spatial modes.

Due to a strong overlap of different modes it is difficult to align thelaser system such that only a substantial portion of a single mode isreflected back into the laser cavity without also reflecting substantialportions of other modes. However, a selective reflection of a singlemode is desirable in order to provide a constructive feedback to asingle mode only and to suppress all other modes. This would result inthe output of a substantial portion of the light intensity within onemode and, therefore, good spatial coherence properties.

In the present invention it is realized that the above problems can beovercome when a laser system, preferably for use in an image setter, thelaser system comprising

-   -   a laser diode adapted to emit a first light beam, the first        light beam comprising a plurality of spatial modes where each        spatial mode has a mode number assigned to it,    -   selection means adapted to select a predetermined part of the        first light beam, and    -   a reflector where the laser diode and the reflector define a        cavity and where the reflector is adapted to reflect the        selected part of the first light beam back into the laser diode        as a second light beam,        is characterised in that the selection means is adapted to        select a part of the first light beam corresponding to a spatial        mode with a higher mode number than a spatial mode with maximum        gain.

Hence, the invention is based on the result that a selection of aspatial mode with a high mode number yields an output beam whichcomprises a high percentage of the output power of the laser diode in anarrow lobe and which may be focused to a small spot size even overlarge distances. This is because the modes with high mode numbers andlow gain are very sensitive to the feedback provided and, therefore, asingle mode is easier to separate from its adjacent modes by providing aselective feedback, thereby yielding a coherent output beam.

According to a preferred embodiment of the invention, the selectionmeans is adapted to substantially select a single spatial mode. When thelaser system emits a high percentage of the output power of the laserdiode in a single spatial mode, the coherence properties of the outputbeam of the laser system are further improved.

In an advantageous and preferred embodiment the mode which is fed backinto the laser is one which normally is not lasing in the free-runninglaser, i.e. which is substantially not present in the first light beamwhen the laser diode is operated without the reflector. Experimentsperformed by the inventor have surprisingly shown that the output beamgenerated by a laser system according to this embodiment of theinvention may be particularly well focused and at the same timecomprises a high optical power.

In a preferred embodiment of the invention the selection means comprisesa first spatial filter placed within the cavity defined by the laserdiode and the reflector. The position and aperture of first spatialfilter may be adapted to only allow the transmission of substantiallyone spatial mode. The first spatial filter may be a slit comprising twosharp edges.

The position and the aperture of the first spatial filter may beadjusted, e.g. by a piezo element.

It is understood that different selection means may be provided. Forexample, the reflector may be a narrow stripe mirror the edges of whichdefine a spatial filter, since only the part of the first light beampropagating in the direction of the mirror is reflected back into thelaser diode.

According to a preferred embodiment of the invention the system furthercomprises a system of optical lenses which creates a pseudo far-field asan image of the Fourier plane of a collimating lens.

In a preferred embodiment the selection means may be positionedsubstantially in said Fourier plane. It is an advantage of thisembodiment that the spatial modes are well-separated in the Fourierplane, and may, therefore, easily be separated by the selection means.

When the system further comprises a second spatial filter adapted toonly allow the transmission of a part of the first light beam which isnot selected by the selection means, the output beam of the laser systemis filtered. Thereby, a considerable noise reduction is achieved andundesired spatial modes are further suppressed, thereby furtherimproving the M₂ value of the output beam. Preferably, the secondspatial filter is placed substantially in the Fourier plane.

When the reflectivity of the output facet of the laser diode is between0.01% and 4%, for example 0.2%, a further increase of the output powerof the laser system may be achieved.

In a preferred embodiment the reflectivity of the output facet isadjusted to correspond to the mode which is fed back into the laserdiode.

The laser diode may be a laser diode array, a broad area laser, a laserdiode bar, a stacked laser array or the like.

When the system further comprises an optical element in the externalcavity, the optical element having a transmittance which is smaller than100%, the amount of feedback provided to the laser cavity may beadjusted and optimised corresponding to the mode which is fed back intothe laser diode.

When the transmittance of the optical element is adjustable, the amountof feedback may continuously be adjusted in order to optimise thequality and power of the output beam.

The reflector may be a mirror, a grating, or the like. The reflector mayalso be the output facet of the laser diode, where the output facet isnot completely parallel to the rear facet of the laser diode. Theposition and orientation of the reflector may be adjusted by a piezoelement.

It is an advantage of the invention that it comprises low-costcomponents which can be arranged in a compact design.

It is an advantage of the invention that it provides a laser system witha high pointing stability, as it eliminates variations in the directionof the output beam caused by a phenomenon called spatial modecompetition. This phenomenon occurs, when the laser operates in aplurality of competing modes with different output angles. If thedominant mode of the laser changes over time, the output angle changesand, thus, the pointing stability decreases. However, when the laser isforced to operate in a single mode, this problem is overcome resultingin a high pointing stability.

According to a second aspect of the invention the mentioned objects areachieved by a method of aligning a laser system for emitting an outputbeam where the laser system comprises

-   -   a laser diode adapted to emit a first light beam along an axis,        the first light beam comprising a plurality of spatial modes        where each spatial mode has a mode number assigned to it,    -   selection means adapted to select a predetermined part of the        first light beam, and    -   a reflector, where the laser diode and the reflector define a        cavity and where the reflector is adapted is to reflect the        selected part of the first light beam back into the laser diode        as a second light beam;        the method comprising the step of aligning the selection means        such that it selects a part of the first light beam        corresponding to a spatial mode with a higher mode number than a        spatial mode with maximum gain.

According to a preferred embodiment of the invention the selection meanscomprises a first spatial filter placed within the cavity and comprisingan inner edge with adjustable position proximal to the axis and an outeredge with adjustable position distal to the axis; the method furthercomprising the steps of

-   -   measuring a predetermined property of the first light beam;    -   adjusting the position of the inner edge of the first spatial        filter until the predetermined property has a predetermined        value;    -   adjusting the position of the outer edge of the first spatial        filter until the predetermined property is substantially        optimised.

It is an advantage of this method of aligning that substantially asingle spatial mode of the laser diode may be selected. This method isparticularly suitable when the single mode is a mode with a high modenumber which is not present in the emission of the free running laser.When the position of the inner edge of the first spatial filter isadjusted, the lower order spatial modes which correspond to a smallangle from the centre axis are suppressed and the lasing of a spatialmode with a high mode number which is not present in the free-runninglaser may effectively be induced.

In a preferred embodiment the predetermined property is the shape of theintensity distribution of the first light beam. In a further preferredembodiment the predetermined shape is characterised by a narrowfar-field angle of the output beam.

According to another preferred embodiment of the invention, thepredetermined property is a measure of the ability of the output beam tobe focused, preferably the M² value of the output beam.

According to another preferred embodiment of the invention the lasersystem comprises an adjustable optical element adapted to control theenergy of the second light beam; and the method comprises the steps of

-   -   measuring a characteristics of the output beam;    -   adjusting the adjustable optical element until the        characteristics has a predetermined value.

It is an advantage of the invention that the output laser beam of thelaser system has good spatial coherence.

It is a further advantage of the invention that the laser system mayeasily be aligned and that the system may be kept aligned.

In a preferred embodiment the adjustable optical element is an opticalelement with adjustable transmittance or a spatial filter withadjustable aperture.

In another preferred embodiment the characteristics is the power of thelaser beam, the angular power density of the laser beam, or the spatialcoherence of the laser beam.

The invention will be explained more fully below in connection withpreferred embodiments and with reference to the drawings, in which:

FIG. 1 shows an application of the invention in the graphical industry;

FIG. 2 shows a schematic view of a first embodiment of the presentinvention;

FIG. 3 shows a schematic view of a second embodiment of the presentinvention;

FIG. 4 shows the far-field power distribution of the embodiment of FIG.3;

FIG. 5 shows the light/current characteristic of the embodiment of FIG.3;

FIG. 6 shows the M² measurement of the output of the embodiment of FIG.3;

FIG. 7 shows a schematic view of a third embodiment of the presentinvention;

FIG. 8 shows a schematic view of a fourth embodiment of the presentinvention; and

FIG. 9 shows a flow diagram of a method of aligning a laser systemaccording to an embodiment of the invention.

FIG. 1 schematically shows an application of a laser system according tothe present invention in an internal drum image setting machine. Thematerial to be exposed, for example a print plate is mounted on theinner surface of a cylinder 1. A laser beam 3 is emitted by a laserdiode system 2 according to the present invention along the axis 7 ofthe cylinder 1. The beam 3 is focused by a lens 4 and reflected by arotating mirror 5 such that the beam 3 is focused as a small spot 6 onthe inner surface of the cylinder 1. The mirror 5 may rotate at a highspeed, for example 30,000 rotations per minute, around the cylinder axis7. At the same time the mirror is moved along the axis 7. In order toachieve a fast exposure of the entire film, a strong laser beam 3 isrequired that can be focused over a distance that at least correspondsto the radius of the cylinder 1. The radius of the cylinder 1 may be 200mm and is determined by the size of the material to be exposed. In orderto achieve high quality printing, the size of the spot 6 should besmall. If the laser system 2 was a known laser diode with an M² value offor example 50, the lens 4 would have to be substantially larger thatthe diameter of the cylinder 1 in order to be able to focus the laserbeam as a small spot 6 on the inner surface of the cylinder 1.

FIG. 2 schematically shows a first embodiment of the present invention.An external feedback configuration comprises a laser diode 21 and aplanar or spherical mirror 23. The optical power emitted by thefree-running laser diode has a far-field angular intensity distributionin the plane of the drawing similar to the one shown as curve 42 in FIG.4. The distribution is a superposition of a plurality of twin-lobedspatial modes. In FIG. 2, the mirror 23 reflects a part of one far-fieldlobe of a spatial mode, which corresponds to an angle θ, back into thelaser diode. Preferably, the spatial mode may be selected by a spatialfilter, such as an adjustable slit, placed between the laser diode 21and the mirror 23. The laser diode 21 and the mirror 23 define anexternal cavity. The feedback from this cavity results in abi-directional beam propagation 22 between the laser diode 21 and themirror 23. Consequently, light of the spatial mode corresponding to theangle θ is fed back into the lasing cavity of the laser diode 21. As aresult of this feedback, the laser diode 21 is seeded to predominantlyoscillate in the spatial mode corresponding to θ, and the second lobe 25of the corresponding twin-lobe structure is emitted at an angle θopposite to a centre plane 24 perpendicular to the low-coherency axis ofthe laser diode 21. If the cavity is adapted to provide feedback for ahigh-order mode which is not present in the free-running laser, theoutput beam has an intensity distribution with a sharp peakcorresponding to a single spatial mode with a small value of M

FIG. 3 schematically shows a second embodiment of the invention. Itshows a laser system with external optical feedback. The systemcomprises a 4 W laser diode 301 with a light emitting aperture of 2×200μm. The output facet of the laser diode is coated with an antireflectioncoating providing a reflectivity of 0.2%. The laser emits a light beamalong an axis 312 comprising a plurality of spatial modes, where eachmode has a far-field twin-lobe intensity distribution. One of thefar-field lobes 305 is reflected back into the laser diode 301 by afirst mirror 306 and the other far-field lobe 302 is extracted from theexternal cavity by a second mirror 303. The output beam 311 is focusedby a lens 310 and may be analysed by a beam analyser (not shown) indifferent distances from the focal plane of the lens 310. A slit 304formed by two razor blades is placed within the cavity defined by thelaser diode 301 and the mirror 306. The position and the aperture of theslit may be adjusted so as to select a far-field lobe corresponding to aspatial mode with a high mode number. For example, the razor bladesforming the slit may be moved relative to each other in order to adjustboth the position and the opening of the slit. Hence, the spatial modeselected by the slit 304 is reflected back by the mirror 306 into thelaser diode 301 where it is amplified by the active medium of the laserdiode 301. Consequently, the selected mode experiences a high gain, andthe other modes are effectively suppressed. The other lobe 302 of thetwin-lobe distribution of the selected mode may thus be extracted fromthe cavity by the mirror 303. The system further comprises two lenses L1and L2, which generate a pseudo far-field in the plane of the slit 304.Between the lenses L1 and L2, a wedge 309 is inserted which extracts apart of the laser beam for diagnostic purposes. A part of the laser beam302, for example 10%, may be used to measure the beam properties indifferent distances from the far field.

FIG. 4 shows the far-field power distribution of the output beam 311 ofthe embodiment of FIG. 3. The spatial characteristics of the lightemitted from broad area lasers or laser diode arrays are known tocomprise a superposition of a plurality of spatial modes. Each of thesemodes has a tendency to emit the optical power in a twin-lobedfar-field, where the two lobes each are centred around an angle +/−θ,respectively, relative to a centre plane perpendicular to thelow-coherency axis of the laser, corresponding to θ=0°. The value of θdepends upon the mode number, and larger mode numbers correspond tolarger values of θ. However, the angular distribution of adjacent modesis strongly overlapping and, consequently, the typical total far-fielddistribution of a high-power laser diode may approximately resemble auniform energy distribution within a range of angles.

Furthermore, it is well-known that there is a spatial mode correspondingto a mode number N_(max), where N_(max) may for example be equal to 10,which experiences a maximum gain in the internal cavity of the laserdiode. Modes with mode numbers smaller and larger than N_(max)experience increasingly weaker gain compared to the mode N_(max).Especially higher modes are strongly suppressed and consequently thetypical total far field distribution of a laser array operated far abovethreshold may resemble a uniform energy distribution in a rangeθ≅θ_(max) to +θ_(max), where θ_(max) may for example be 3°. FIG. 4 showsmeasured far-fields from the laser diode with (curve 41) and without(curve 42) optical feedback, respectively. The wavelength of the outputbeam is 804 nm, and the laser diode is operated at 1.7 A. With feedbackthe radiation angle in the far-field is decreased by approximately afactor of 10 with almost all the optical power, up to 90%, contained inthis state with high spatial coherence. The optimal peak of the powerdistribution with feedback 41 is situated at a large distance from thecentre of the distribution 42 of the free-running laser. Thiscorresponds to a spatial mode with high mode number and large angle θ.It is noted that the distribution further comprises a small peak 43around an angle −θ corresponding to the lobe of the selected spatialmode in the direction of the feedback cavity.

FIG. 5 shows the light-current characteristics of the embodiment of FIG.3. The optical power versus diode current is shown in the figure with(curve 52) and without (curve 51) feedback, respectively. One importanteffect, caused by the antireflection coating of the output facet, isthat the threshold of the laser is reduced by 0.3 A when the feedback ison. Furthermore it is experimentally observed that the slope efficiencyis 0.81 with external feedback. This means that an increase of the diodecurrent by 1 A results in an increase of the optical power by 0.81 W.The slope efficiency without feedback is 0.84. This shows that the diodewith external feedback is operated with a very high efficiency in a modewith high spatial coherence.

FIG. 6 shows the M² measurement of the output of the embodiment of FIG.3 along the low-coherency axis. The M² value is related to the abilityof a light source to be focused compared to a theoretical optimum, i.e.a light source with a Gaussian intensity distribution. The measurementof M² does not only take the beam properties in the far field intoaccount but rather at different distances from the focal plane. If, forexample, a light source has an M² value of 3, the light beam of thissource is referred to as being 3 times diffraction limited. The qualityparameter M² has been measured along the low-coherency axis as shown inFIG. 6 and along the high-coherency axis (not shown). When the feedbackis on, the M² of the low-coherency axis of the diode is decreased to avalue of 1.8. The M₂ value of the high-coherency axis of the laser diodeis measured to 1.9. Since the high-coherency axis of the diodecorresponds to an almost diffraction limited spatial mode these resultsshow that the low-coherency axis is improved to almost the fundamentallimit by the optical feedback.

FIG. 7 schematically shows a third embodiment of the present invention.It shows a laser system 701 with external feedback comprising a broadarea 4 W GaAlAs gain-guided laser 702. The laser emits a light beam 714comprising a plurality of spatial modes, where each mode has a far-fieldtwin-lobe intensity distribution perpendicular to the plane of thedrawing. One of the far-field lobes 705 is reflected back into the laserdiode 702 by a grating 707 and the other far-field lobe is extractedfrom the external cavity by a mirror 704. The mirror 704 is adapted toonly reflect a part of the light beam 714 which is not transmittedthrough the spatial filter 706. This filtering of the output beam 712improves the spatial coherence of the output beam by reducing the effectof background noise. The output beam 712 is transmitted via a secondmirror 703 through a beam expander 713 consisting of two cylindricallenses. The purpose of this beam expander is to transform the outputbeam 712 into a circular beam. According to the invention, a spatialmode with high mode number is selected by a slit 706 formed by two razorblades. The system further comprises a spherical lens 708 and twocylindrical lenses 710 and 711, which generate a pseudo far-field in theplane of the slit 706.

FIG. 8 schematically shows a forth embodiment of the present invention.It shows a laser system with external optical feedback. The systemcomprises a 4 W laser diode 801 with a light emitting aperture of 2×200μm. The laser emits a light beam 802 comprising a plurality of spatialmodes, where each mode has a far-field twin-lobe intensity distributionperpendicular to the plane of the drawing. One of the far-field lobes805 is reflected back into the laser diode 801 by a mirror 806 and theother far-field lobe is extracted from the external cavity by a mirror803. According to the invention, a spatial mode with high mode number isselected by a slit 804 formed by two razor blades, which may be movedrelative to each other. The system further comprises two lenses 807 and808, which generate a pseudo far-field in the plane of the slit 804. Thesystem further comprises a wedge 8.15 which extracts a small portion,for example 5%, of the output beam 811. A collimating lens 810 focusesthe extracted light beam 816. A beam analyser 814 is placed in the focalplane of the lens 810. The beam analyser measures the intensity profileof the extracted beam 816. The measured signal is digitally analysed inan analysis unit 809, where the width of the dominant peak of themeasured intensity profile is calculated. Based on the calculated width,a control unit 813 controls a piezo element 812 to adjust the size ofthe aperture of the slit 804 until the width of the dominant peak isminimized.

FIG. 9 shows a flow diagram of a method of aligning a laser system on anoptical table according to an embodiment of the invention. In thefollowing reference is also made to the system illustrated in FIG. 3.

Setting up the high power laser diode (Step 901): A single broad stripelaser diode 301 with stripe width 25–700 μm is placed on an opticaltable with its junction perpendicular to the table plane. In FIG. 3, thetable plane is orthogonal to the plane of the drawing. Thehigh-coherence axis of the laser diode is in the horizontal directionand the low-coherence axis is in the vertical direction.

Measuring the intensity profile in the far field (step 902): Thefar-field intensity profile of the laser diode 1–2 m after the laserdiode is measured in the vertical direction. The current to the laser isvaried to I=1.1 I_(th); I=2 I_(th); I=3 I_(th) (I_(th) being thethreshold current of the laser diode) until the maximum current isobtained, and for each current setting the far field intensitydistribution is measured.

Generation of a pseudo far-field at the position of the spatial filter(step 903): The far-field intensity profile is now reconstructed at theposition of the spatial filter using two lenses L1 and L2. The laser maybe operated at I=1.5 I_(th) during this procedure. A lens L1 with asmall focal length f_(L1), e.g. f_(L1)≈3–5 mm, is placed right in frontof the laser. This short focal length lens collimates the beam in thehorizontal plane. A cylindrical lens L2 is used to collimate the beam inthe vertical direction. A low reflection beam splitter wedge 309 isplaced after L2. Alternatively, the wedge 309 may be placed between L1and L2 as illustrated in FIG. 3. The wedge provides two reflectionswhich can be used to measure the intensity profile and the frequencyspectrum of the laser diode, respectively.

The spatial filter is now placed in the pseudo far-field plane. Theposition of the pseudo far field plane may be estimated using the lensmaker's equation:1/f=1/S ₁+1/S ₂.

At the estimated far-field the intensity profiles are now recorded forthe current settings I=1.1 I_(th); I=2 I_(th); I=3 I_(th) until themaximum current. These intensity profiles are now compared with theintensity profiles in the real far field and if discrepancy occur theposition of the spatial filter is changed until the intensity profilesof the real far-field are completely reproduced.

Measurering the intensity profile using the first reflection from thewedge (step 904): The far field plane of the first reflection from thewedge 309 is estimated by inserting a convex lens. A beam analyser isplaced in the far-field plane.

Establishment of the asymmetric feedback into the diode laser (step905): A mirror 306 or a reflection grating is placed right behind thespatial filter 304. Both components should substantially be placed inthe far-field plane. The mirror 306 should be oriented so that theoutput is directed exactly back into the laser diode 301. If a gratingis used instead of the mirror it should be aligned such that the firstdiffraction order is directed back into the laser. The mirror or thegrating is adjusted until the intensity profile monitored by the beamanalyser is changed significantly.

Adjusting the spatial filter until a single-lobed output beam with lowM2 is obtained (step 906): The spatial filter is now fully open totransmit the feedback beam. Subsequently, the centre part 304 a of thespatial filter which is proximal to the centre of the angular intensitydistribution of the laser beam, in the above example the razor blade 304a, is slowly inserted to cut the centre part of the beam. This inneredge of the spatial filter is adjusted towards a distal position wherethe diode laser is forced to operate in a spatial mode with high modenumber which normally is not present in the free running laser. When thecentre part of the filter is adjusted, the distal part 304 b of thefilter is inserted and adjusted. The intensity profile shown on the beamanalyzer should be observed when the filtering is performed. When thefilter is correctly adjusted, the intensity profile is narrowed down andits shape changes from a broad distribution to a double lobeddistribution comprising a small and a large peak as illustrated by FIG.4. Preferably, the chosen spatial mode corresponding to the double lobeddistribution is situated at one of the edges of the broad spatialdistribution.

A mirror 303 is now inserted in the pseudo far-field plane and thismirror couples out the large intensity peak of the doubled lobeddistribution. The M² of the output beam is now monitored using a M² beampropagation analyser. The spatial filter in the feedback circuit isadjusted carefully until the M²-value is minimized.

A spatial filter placed in the pseudo far-field is now used to filterthe output lobe. The output filter is now adjusted until the M²-value isfurther minimized. As an example a so-called half-mirror with a sharpcutting edge may be used for simultaneously filtering and coupling outthe output beam. The purpose of the filtering of the output beam is toremove noise from the single lobed output beam. This noise hassignificant influence on the M²-value.

1. A laser system for an image setter, comprising: a laser diode adaptedto emit a first light beam, the first light beam having a spatialintensity distribution comprising a plurality of spatial modes whereineach spatial mode has a mode number assigned to it, and wherein eachspatial mode has a twin-lobed intensity distribution including a firstand a second lobe, selection means adapted to select a first lobe of atleast one of the twin-lobed intensity distribution; a reflector thelaser diode and the reflector defining a cavity and the reflector beingadapted to reflect the selected first lobe back into the laser diode asa second light beam, thereby providing the second lobe of the at leastone twin-lobed intensity distribution as an output beam; the selectionmeans being adapted to select the first lobe corresponding to a spatialmode having a higher mode number greater than a spatial mode withmaximum gain; and wherein the selection means is adapted to select aspatial mode which is substantially not present in the first light beamwhen the laser diode is operated without the reflector.
 2. The lasersystem according to claim 1, wherein the selection means is adapted tosubstantially select a single spatial mode.
 3. The laser systemaccording to claim 1, further including at least one optical lens placedwithin the cavity defined by the laser diode and the reflector, and theselection means is located substantially in the Fourier plane of the atleast one optical lens.
 4. The laser system according to claim 1,wherein the laser diode comprises a light-emitting facet with areflectivity substantially between 0.01% and 4%.
 5. The system accordingto claim 4, wherein the reflectivity of the light emitting facet is0.2%.
 6. The laser system according to claim 4, wherein the reflectivityof the light emitting facet is adjusted to correspond to the spatialmode of the second light beam.
 7. The laser system according to claim 1,further including an optical element inside the cavity, the opticalelement having a transmittance of less than 100%.
 8. The laser systemaccording to claim 7, wherein the transmittance of the optical elementis adjustable.
 9. The laser system according to claim 1, wherein thelaser diode is a laser diode bar.
 10. The laser system according toclaim 1 wherein the laser diode is a stacked laser array.
 11. A lasersystem for an image setter, comprising: a laser diode adapted to emit afirst light beam, the first light beam having a spatial intensitydistribution comprising a plurality of spatial modes wherein eachspatial mode has a mode number assigned to it, and wherein each spatialmode has a twin-lobed intensity distribution including a first and asecond lobe, selection means adapted to select a first lobe of at leastone of the twin-lobed intensity distribution; a reflector; the laserdiode and the reflector defining a cavity and the reflector beingadapted to reflect the selected first lobe back into the laser diode asa second light beam, thereby providing the second lobe of the at leastone twin-lobed intensity distribution as an output beam; the selectionmeans being adapted to select the first lobe corresponding to a spatialmode having a higher mode number greater than a spatial mode withmaximum gain; wherein the selection means comprises a first spatialfilter placed within the cavity defined by the laser diode and thereflector; and wherein the first spatial filter has an aperture ofadjustable size.
 12. The laser system according to claim 11, wherein thefirst spatial filter has an adjustable position.
 13. A laser system foran image setter, comprising: a laser diode adapted to emit a first lightbeam, the first light beam having a spatial intensity distributioncomprising a plurality of spatial modes wherein each spatial mode has amode number assigned to it, and wherein each spatial mode has atwin-lobed intensity distribution including a first and a second lobe,selection means adapted to select a first lobe of at least one of thetwin-lobed intensity distribution; a reflector; the laser diode and thereflector defining a cavity and the reflector being adapted to reflectthe selected first lobe back into the laser diode as a second lightbeam, thereby providing the second lobe of the at least one twin-lobedintensity distribution as an output beam; the selection means beingadapted to select the first lobe corresponding to a spatial mode havinga higher mode number greater than a spatial mode with maximum gain; andfurther including a second spatial filter adapted to allow thetransmission of a part of the first light beam not selected by theselection means.
 14. A method of aligning a laser system for emitting anoutput beam where the laser system comprises: a laser diode adapted toemit a first light beam along an axis, the first light beam having aspatial intensity distribution comprising a plurality of spatial modeswhere each spatial mode has a mode number assigned to it, and whereineach spatial mode has a twin-lobed intensity distribution including afirst and a second lobe, selection means adapted to select a first lobeof at least one of the twin-lobed intensity distributions, and areflector where the laser diode and the reflector define a cavity andwhere the reflector is adapted to reflect the selected first lobe backinto the laser diode as a second light beam thereby providing the secondlobe of the at least one twin-lobed intensity distribution as an outputbeam, the method comprising the step of: aligning the selection meanssuch that it selects the first lobe corresponding to a spatial mode witha higher mode number than a spatial mode with maximum gain wherein theselection means comprises a first spatial filter placed within thecavity and comprising an inner edge (304 a) with an adjustable positionproximal to the axis and an outer edge (304 b) with an adjustableposition distal to the axis the method further comprising the steps of:measuring a predetermined property of the first light beam adjusting theposition of the inner edge of the first spatial filter until thepredetermined property has a predetermined value adjusting the positionof the outer edge of the first spatial filter until predeterminedproperty is substantially optimized.
 15. The method according to claim14, wherein the predetermined property is a measure of the ability of atleast a part of the first light beam corresponding to the output beam tobe focused.
 16. The method according to claim 15, wherein thepredetermined property is an M² value of the output beam.
 17. The methodaccording to claim 14, wherein the predetermined property is a shape ofan intensity distribution of the first light beam.
 18. The methodaccording to claim 17, wherein the step of adjusting the position of theinner edge further includes adjusting the position of the inner edge ofthe first spatial filter until an angular intensity distribution of theoutput beam has a minimal far-field angle.
 19. A method of aligning alaser system for emitting an output beam where the laser systemcomprises: a laser diode adapted to emit a first light beam along anaxis, the first light beam having a spatial intensity distributioncomprising a plurality of spatial modes where each spatial mode has amode number assigned to it, and wherein each spatial mode has atwin-lobed intensity distribution including a first and a second lobe,selection means adapted to select a first lobe of at least one of thetwin-lobed intensity distributions, and a reflector where the laserdiode and the reflector define a cavity and where the reflector isadapted to reflect the selected first lobe back into the laser diode asa second light beam thereby providing the second lobe of the at leastone twin-lobed intensity distribution as an output beam, the methodcomprising the step of: aligning the selection means such that itselects the first lobe corresponding to a spatial mode with a highermode number than a spatial mode with maximum gain wherein the step ofaligning the selection means further includes the step of aligning theselection means such that the selection means selects a spatial modewhich is substantially not present in the light beam when the laserdiode is operated without the reflector.
 20. The method according toclaim 19, wherein the laser diode comprises a light-emitting facet witha reflectivity between 0.01% and 4%.
 21. The method according to claim20, wherein the reflectivity of the light-emitting facet is 0.2%. 22.The method according to claim 19, wherein the system further includes anoptical element inside the cavity, the optical element having atransmittance which is smaller than 100%.
 23. The method according toclaim 19, wherein the system further includes at least one optical lensplaced inside the cavity, and the first spatial filter is locatedsubstantially in the Fourier plane of the at least one optical lens. 24.The method according to claim 19, wherein the system further comprises asecond spatial filter adapted to transmission of a second part of thelight beam which is not transmitted through the first spatial filter.25. The method according to claim 19, wherein the laser diode is a laserdiode bar.
 26. The method according to claim 19 wherein the laser diodeis a stacked laser array.
 27. The method according to claim 19 whereinthe laser system further includes an adjustable optical element adaptedto control the energy of the second light beam, and the methodcomprising the steps of: measuring a characteristic of an output laserbeam of the laser system; and adjusting the adjustable optical elementuntil the characteristic has a predetermined value.
 28. The methodaccording to claim 27, wherein the adjustable optical element is anoptical element with adjustable transmittance.
 29. The method accordingto claim 27, wherein the adjustable optical element is a spatial filterwith an adjustable aperture.
 30. The method according to claim 29,wherein the characteristic is the spatial coherence of the output beam.31. The method according to claim 27, wherein the characteristic is thepower of the output beam.
 32. The method according to claim 27, whereinthe characteristic is the angular power density of the output beam. 33.A method of aligning a laser system for emitting an output beam wherethe laser system comprises: a laser diode adapted to emit a first lightbeam along an axis, the first light beam comprising a plurality ofspatial modes where each spatial mode has a mode number assigned to it,selection means adapted to select a predetermined part of the firstlight beam, and a reflector where the laser diode and the reflectordefine a cavity and where the reflector is adapted to reflect theselected part of the first light beam back into the laser diode as asecond light beam, the method comprising the step of: aligning theselection means such that it selects a part of the first light beamcorresponding to a spatial mode with a higher mode number than a spatialmode with maximum gain, wherein the step of aligning the selection meansfurther includes the step of aligning the selection means such that theselection means selects a spatial mode which is substantially notpresent in the light beam when the laser diode is operated without thereflector.